Microphone employing piezoresistive element



y 14, 1968 A. M. WIGGINS MICROPHONE EMPLOYING PIEZORESISTIVE ELEMENT Filed Sept. 8, 1965 INVENTOR ALPHA M. Wmenvs BY 6 ATTORNEY FIG ax 5% 805 L\\\\ United States Patent Office 3,383,475 Patented May 14, 1968 3,383,475 MECROPHONE EMPLOYING PIEZORESISTIVE ELEMENT Alpha M. Wiggins, San Juan, Puerto Rico, assignor to Euphonics Corporation, Guaynabo, Puerto Rico, a corporation of Puerto Rico Filed Sept. 8, 1965, Ser. No. 485,760 11 Claims. (Cl. 179-110) This invention concerns an improved micro-phone employing a piezoresistive element.

The invention is particularly directed at a microphone including a semiconductor element which has piezoresistive characteristics. Such an element changes in electrical resistance or conductance when subjected to a mechanical stress or strain. A particular difficulty encountered inusing such an element in a microphone is that the piezoresistive element is necessarily very small and is thus very delicate and fragile. An-y violent or excessive force applied thereto results in fracture of the element if it is not properly mounted. The invention is therefore further directed at providing an improved safety mounting for a piezoresistive element in a microphone.

Heretofore it has been conventional in microphones to employ transducers which convert mechanical energy into electrical energy. The mechanical energy is derived from compressional waves applied to a diaphragm which vibrate the diaphragm and in turn the transducer or transducers coupled to the diaphragm. The conventional transducers used in diaphragms are active electrical generatin elements which are generally piezoelectric or electromagnetic. These types of transducers have a number of difiiculties and disadvantages particularly when associated with transistor amplifier circuits. Some transducers such as crystals and ceramics of various types exhibiting piezoelectric characteristics, when used in microphones produce voltages proportional to diaphragm excursions. These piezoelectric elements are high impedance, capacitive elements poorly adapted to work into low impedance loads such as transistor inputs.

Further limitations experienced in prior conventional microphones employing transducers acting as electrical generators are: mismatch of transistor circuitry so as to produce insignificant power at low impedance-s; excessive susceptibility to stray magnetic or electrostatic fields so as to produce hum; need for complicated high gain preamplifiers; mechanically fragile and subject to degradation and deterioration at extremes of temperature and humidity; complex and elaborate mounting systems required for the transducer elements; and many other limitations which multiply ias demands increase for the high-est fidelity of pickup of audio signals from impinging sound waves.

The above mentioned and other difiicul-ties and disadvantages of prior microphones are avoided by the present invention in which the piezoresistive transducer serves as a passive element responding only by a change of electrical resistance or conductance as the associated coupled diaphragm applies a mechanical stress or strain. An external electrical source applies electric current to the transducer so that the transducer serves as a modulator rather than :a voltage generator. As a result an electrical response is obtainable from the piezoresistive transducer which maybe 100 or more times greater in amplitude or energy content than the response obtainable from a piezoelectric or magnetic transducer.

The microphone embodying the invention employs a transducer which has a piezoresistive semiconductor element made of silicon activated by inclusion of an appro priate impurity such as copper, silver, gold, etc. The material of the semiconductor may be of a type employed in the manufacture of silicon transistors. Such a material when properly activated to render it conductive electrioally will have piezoresistive properties. While use of such material is known in transistors, this semiconductor material has not heretofore been used in transducers of microphones. It is possible in practicing the invention to use other known types of piezoresistive material than activated silicon, the precise composition of such material being beyond the scope of the present invention and known in the art.

An important feature of the invention is the provision 'of a notched beam on which an activated piezoresistive element overlays the notch in the beam. A central, thin portion of the beam at the notch serves as a hinge. A resilient, compliant loop encircles the beam and piezoresistive element at the hinge of the beam. A compliant link integral with the loop is connected to the center of the diaphragm which is mounted to the periphcry of the case of the microphone. Opposite ends of the beam are inserted in compliant, resilient cups which fit fric'tionall-y in stationary receptacles or sockets mounted to the case of the microphone. The cups are not secured to the sockets and are only frictionally engaged therein. If an excessive force is applied to the diaphragm inwardly axially of the link and perpendicularly to the beam, this force will not injure the beam and piezoresistive element because the compliant link will flex. If the excessive force is directed outwardly of the axis of the link and diaphragm, the link will first stretch and then the beam will move as it is pulled physically outwardly as the compliant cups slide frictionally inthe sockets. Actually the total excursion of the beam even on a severe outward deflection of the diaphragm will only be a few thousandths of an inch (mils), so that on relief of the outward deflecting force, the beams and cups will remain in place within the sockets, albeit slightly displaced from their back surfaces. This will not imp-air the future operation of the microphone [and may even be of some advantage since there will now be a slight space between each compliant cup and the back surface of its socket into which the cup can move in the event of a subsequent severe inward deflection of the beam. Thus the delicate piezoresistive element is effectively protected against untoward mechanioal shocks which otherwise would render it inoperative.

It is therefore a principal object of the invention to provide a novel microphone employing a piezoresistive element.

A further object is to provide a microphone of simplified construction having a piezoresistive element with a floating mounting and comspliantly coupled to a diaphragm, so that the element is effectively protected against violent excessive excursions of the diaphragm.

Other objects and advantages of the invention will become apparent from the following detailed description taken together with the drawing, wherein:

FIG. 1 is a top plan view on an enlarged scale of a microphone embodying the invention.

FIG. 2 and FIG. 3 are sectional views taken on lines 22 and 33 respectively of FIG. 1.

FIG. 4 is a further enlarged fragmentary sectional view taken on line 4-4 of FIG. 1.

FIG. 5 is an exploded perspective view showing parts of the microphone, part of the case being broken away.

FIG. 6 is an oblique rear and side perspective view of the microphone on a scale reduced from that of FIGS. 1-3.

FIG. 7 is a diagram of an electric circuit employed in explaining the invention.

Referring first to FIGS. 16-, there is shown a microphone having a dished case 1% with a circular closed back 12 and generally cylindrical side wall 14. An annular flange 11, extending radially outward of the cylindrical wall 14, is formed at the front free edge of this wall. Notches are used to receive screws for mounting the microphone on a suitable support.

The microphone further includes a generally conical diaphragm 16 made of thin, tough, dimensionally stable plastic material. The diaphragm has an outer annular flange 17 which can be secured by a suitable cement or other adhesive 18 to the outer side of flange 16. Radial ribs 20 can be formed in the diaphragm to stiffen it and to damp undesired spurious vibrations of the diaphragm. The diaphragm has a flattened apical end 22 in which is formed a hole 23. In this hole engages a tip 24 of a resilient, compliant coupling member 25.

The coupling member has a straight, flexible link 26 extending axially inward of the case from the apex of the diaphragm. The convex side of the diaphragm faces inwardly of the case as clearly shown in FIGS. 2 and 3. The tip 24 is secured by a suitable adhesive 23 at the hole 23 so that the diaphragm in vibrating axially effectively drives the coupling member 25.

The coupling member has a rectangular loop 30 integral with the inner end of link 26 and completely encircling a piezoresistive transducer 35. The transducer is shown to best advantage in FIGS. 15. The transducer 35 has a rectangular bar or beam 4! made of a solid plastic electrical insulation material such as an epoxy. A notch 42 is formed transversely of the beam and extends inwardly from fiat side 43 toward side 44. The notch terminates short of side 44 to define a narrow, thin bridge 45 which serves as a hinge when the beam is flexed as the coupling member moves axially of the link or arm 26 and of the diaphragm 16. Extending across the open side of the noitch at side 43 of the beam is a piezoresistive semiconductor element 50. It will be noted that this element has a constricted, thin narrow central neck 52 with two wider integral generally rectangular heads 53. The heads are each secured by a suitable solder layer 57 of thin metallic coatings 54, applied to opposite end portions of side 43 of the beam. The coatings 54, 55 terminate at the notch. The loop 3%) encircles the neck 52 and adjacent portions of heads 53; see FIG. 2. Applied to the fiat side 44 of the beam is a metallic layer or coating 56. This layer may be omitted if desired.

Opposite ends of the beam are inserted and frictionally engaged in respective compliant, resilient cups 60a, 60b. These cups are rectangular in cross section with a closed end wall 62 integral with four mutually perpendicular rectangular side walls 66. The cups in turn are frictionally and elastically engaged in rigid, plastic sockets 68a, 68b integrally formed with a base plate '70 at opposite lateral edges thereof. The sockets each have a fiat, rectangular bottom or back wall '72, and three vertical walls. Walls 73 are parallel to each other and perpendicular to end wall 74. These 'walls receive between them opposite vertical walls 66 of the cups 66a, 6% while the end walls 62 of the cups frictionally engage the inner vertical sides of end walls 74. It will be apparent from an inspection of FIGS. 2-5 that the cups 60a, 60b can move vertically upward in the sockets 68a, 68b if sufficient upward pressure is applied thereto. Normally however the beam engaged in the cups is snugly received and gripped in the sockets in a yield-able floating frictional engagement. It has been found by repeated experiments that if either one or both of the compliant cups are secured by cementing or otherwise in the sockets, the beam 40 and/ or the delicate semiconductor element 50 will break when excessive force is applied to the beam axially of the link 26. By the arrangement described, the link will fiex if excessive inward force is directed toward the beam and the link will stretch and draw the beam upwardly in the sockets 68a, 6812.

Electrical connections are made to opposite ends of the element 26 by two light, pliable, metallic strips 86, One end of strip an is inserted inside f p an is 4 frictionally engaged between metallic coating 54 and the adjacent underside of the cup 6%. The other end of strip 20 terminates at a terminal lug 84 to which the strip end is soldered. Lug 84 is secured by a rivet 85 inserted in a hole 86 in plate 70 and in a registering hole 88 formed in recessed portion 12a of the back of the case 10.

One end of conductive strip 82 is inserted inside of cup 68b and is bent around the end of the beam thereat to contact the end of conductive coating 55 in cup 60b; see FIGS. 2 and 5. The other end of strip 82 terminates a lug 89 secured to a rivet 90 inserted through aligned holes 91, 92 in plate 70 and the back 12 of case 10. Hole 92 is enlarged to receive a boss 94 formed on plate 70; and an insulating washer 95 is placed under the head 90' of rivet 99'.

An anchoring pin or projection 96 may be formed at the underside of plate 70. This projection engages in a hole 97 formed in the back of case 16; see FIGS. 3 and 6. Wires W can be soldered to the rivet heads to connect the microphone to an external circuit.

Two circumferential, rectangular openings 98 can be formed in the cylindrical wall of the case. These openings may be covered by protective fiberglass cloth strips 9? which will exclude dust while freely passing air to relieve back pressure on the diaphragm inside case 10 when the diaphragm is vibrating.

FIG. 7 shows in simplified form, a suitable transistor amplifier circuit 196 which may be driven by the microphone. The piezoresistive element 50 is represented schematically by the symbol for a variable resistor in a dotted rectangle. It will be noted that the piezoresistive element 50 is connected to opposite terminals of a battery 102 or other DC. power supply in series with a resistor 104. One end of element 50 is connected via a capacitor 106 to base 108 of a transistor 110. One terminal P1 of the battery is Connected to base 108 via resistor 113. The other terminal P2 of the battery is connected to collector 114 of the transistor. The emitter 119 is connected to output terminal 115 of the circuit via capacitor 117. Battery terminal P2 is connected to output terminal 116. The emitter is also connected to terminal P1 of the battery via bias resistor 120. The base 108 and collector 114- are connected by resistor 12-12.

It will be apparent from an inspection of the amplifier circuit that variations in resistance or conductance of the piezoresistive transducer element 50 caused by applies stresses and strains and releases of the same, cause variations in current flowing through the transducer element. These variations are amplified by the transistor 11!!) and are applied to output terminals 115, 116 as a DC. voltage of varying amplitude. This provides a significant advantage that the microphone is responsive at all frequencies of applied compressional air waves from zero up to the maximum frequency at which the diaphragm is capable of vibrating without introducing undesirable distortion. The transducer element generates no current. It presents a low impedance load to the transistor for etlicient coupling of electrical oscillations thereto. The transistor can thus be driven effectively by a current supplied by its own power supply, modulated by the transducer element 50. Since the transducer serves only to conduct current it can have a very small mass at its neck 52 and can be smaller in width and length than the minimum cross sectional dimension of the driving arm or link 26. This loads the diaphragm very lightly so that it can drive the transducer element positively without itself resonating.

To sum up then, the microphone has among others, the following desirable characteristics:

(1) It has no limitations in response range or smoothness of response within or adjacent to the full audio frequency range.

(2) It employs an electrical modulating system obtaining power from an external source.

(3) It has extremely small mass, i.e., driving impedance, to permit precise response to very small, rapid excursions of the diaphragm of the microphone.

(4) It matches electrically conventional transistor circuitry, to supply significant driving power at an impedance of about 1000 ohms.

(5) It is not susceptible to hum, noise and interference due to induced magnetic or electrostatic fields.

(6) It does not require high gain, complex, expensive amplifiers.

(7) It does not require high cost, complicated mounting arrangements.

(8) It is mechanically rugged and resistant to extremes of temperature and humidity.

(9) The transducer element is effectively protected from Sudden, violent mechanical forces regardless of the direction from which applied.

(10) The microphone basically has a simple structure which makes it possible to manufacture it very economically, at low labor cost, and from relatively inexpensive materials; but the quality of its performance will exceed that of microphones having more complex construction.

While the transducer element is shown in the drawing as having a plastic beam, it is possible to employ a beam made of other materials. For example, the beam could be made of metal. The element 50 and the conductors electrically in circuit with this element would then be electrically insulated by insulation means from the metal beam. Other modification may be made Without departing from the invention as defined by the claims.

What is claimed and sought to be protected by Letters Patent is:

1. A microphone comprising:

a support;

a pair of spaced sockets mounted in a stationary posi tion on said support;

a transducer including:

a beam having a lateral slot at one side thereof,

a piezoresistive element on said one side of the beam and extending over said slot, said slot terminating short of the other side of the beam to define a narrow flexible bridge between opposite ends of the beam, and

compliant support members on opposite ends respectively of the beam, said ends of the beam with the compliant support members thereon being frictionaily engaged in the sockets respectively and movable in the sockets upon application of an abnormally large force directed laterally of the beam, whereby said element and said bridge are protected from mechanical injury.

2. A microphone as recited in claim 1, further comprising:

a compliant coupling member engaging said beam at the bridge for applying bending forces to the beam and said element to change electrical resistance of said element.

3. A microphone according to claim 2, further comprising:

a diaphragm carried by said support, said compliant coupling member being connected to said diaphragm and being moved thereby when compressional Waves are applied to said diaphragm for applying said bending forces to said beam and element.

4. A microphone comprising:

a casing having a closed back Wall and integral generally cylindrical side wall with a free annular edge;

a vibratile diaphragm secured peripherally to the annular edge of the cylindrical side wall;

a compliant coupling member having a flexible, elastic arm secured at one end to said diaphragm, and a loop at the other end of the arm integral therewith; and

a transducer including an elongated flexible beam engaged in said loop at a point spaced from both ends of the beam; a piezoresistive element carried by and secured to said beam, said element having an electrical resistance which changes when said element is flexed upon axial movement of said arm by said diaphragm;

compliant support means on opposite ends of the beam;

and

beam support means carried by the casing and engaging said compliant support means in floating, frictional engagement at each end of the beam.

5. A microphone according to claim 4, further comprising:

electric current conduction means connected electrically to opposite ends of said piezoresistive element and terminating outside the case for connection to an external electric circuit.

6. A microphone according to claim 3 further comprising:

electric current conduction means connected electrical- 1y to opposite ends of said piezoresistive element and terminating outside the case for connection to an external electric circuit.

7. A microphone comprising:

a support;

a transducer including a beam having a piezoresistive element on one side thereof;

means connecting opposite ends of the beam to said support in free, slidable, frictional engagement therewith; and

means for applying bending forces to said element for changing its electrical resistance.

8. A microphone according to claim 7, wherein the last named means comprises:

a compliant coupling member; and

a diaphragm, said coupling member being connected at opposite ends to said diaphragm and said beam for applying said forces to said element when the diapraghm is moved.

9. A microphone according to claim 4, further comprising:

electrical conductors on said beam in direct electrical circuit with opposite ends of said element; and

electric current conduction means electrically connected to said electrical conductors respectively and terminating outside the casing for connection to an external electric circuit.

10. A microphone according to claim 3, further comprising:

a first pair of electric current conductors on said beam in direct electrical circuit with opposite ends of said element, said conductors terminating respectively at points on said beam spaced from said ends of said element; and

a second pair of electric current conductors electrically connected to ends of the first pair of conductors respectively at said points on the beam, said second pair of conductors terminating outside the casing for connection to an external electric circuit.

11. A microphone comprising:

a support;

a transducer including a piezoresistive element;

means connecting opposite ends of said transducer to said support in free, slidable, frictional engagement therewith; and

means for applying bending forces to said element for changing its electrical resistance.

No references cited. KATHLEEN H. CLAFFY, Primary Examiner. A. MCGILL, Assistant Examiner. 

11. A MICROPHONE COMPRISING: A SUPPORT; A TRANSDUCER INCLUDING A PIEZORESISTIVE ELEMENT; MEANS CONNECTING OPPOSITE ENDS OF SAID TRANSDUCER TO SAID SUPPORT IN FREE, SLIDABLE, FRICTIONAL ENGAGEMENT THEREWITH; AND MEANS FOR APPLYING BENDING FORCES TO SAID ELEMENT FOR CHANGING ITS ELECTRICAL RESISTANCE. 